The present invention relates to an improved heat transfer integral tube and strip fin heat exchanger circuit, and to a process for making the integral tube and strip fin heat exchanger circuit, which has particular utility in refrigerant heat transfer.
In refrigeration applications, it is common to utilize a refrigerant-carrying tube to supply the means by which heat is exchanged from the chamber or areas to be conditioned. Ordinarily, the heat removal is accomplished by forced convection between two separated fluids. For example in household refrigerators, air conditioners, or heat pump systems, the two separated fluids would be a refrigerant contained within a tube and air flowing across the refrigerant-carrying tube to assist in transferring heat to or from the tube wall as imparted by the heat of vaporization or condensation of the refrigerant within the tube. In such applications, the refrigerant carrying tubes are usually provided either as a condenser or an evaporator.
In such forced convection applications, it is common practice to provide a balance between the amount of heat transfer surface area and the heat transfer coefficients of the respective surfaces. The air side surface usually has a relatively low heat transfer coefficient; therefore, a greater amount of exposed heat transfer surface is generally provided to maintain this balance. It is also necessary to provide an economic balance between the amount and structure of tile exposed heat transfer surface considering the heat transfer coefficients of the fluids involved. For instance, in a standard refrigeration application, the refrigerant has a significantly greater ability to transfer heat to the tube in which it is carried than does the air which flows thereacross to remove the heat transferred to the tube by the refrigerant. Therefore, it is accepted practice in the refrigeration art to substantially increase the surface area provided on the outside, or air side, of the tube to balance the ability of the refrigerant to supply heat to the inside of the tube. In producing a heat transfer surface it is economically advantageous to attempt to provide the exposed surface area in contact with two fluids in an inverse ratio of the fluids' ability to transfer heat. For this reason, it is accepted practice to add surface area to the air side of the container in the form of fins. Many types of finned tubing are commercially available for use in refrigerant-to air heat exchangers (both evaporators and condensers). One type of extended surface fin known as a "looped fin" as disclosed in my prior U.S. Pat. No. 5,033,544. Another type of extended surface fin known as a "spine fin" is disclosed in my prior U.S. Pat. No. 2,983,300. Plate fins and other types of extended surface fins are disclosed in U.S. Pat. No. 4,143,710 issued to LaPorte et al. These latter fins are complex geometric shapes, which are difficult to fabricate and have a higher degree of wasted material in relation to the heat transfer capacity provided. The spine fin is mechanically weak and has a low resistance to bending and compressive forces; therefore, to permit practical utilization of the spine fin, in use the spine fins are spaced or bunched very closely on the refrigerant tube.
Another disadvantage of the spine fin heat transfer device is that the thin strips of the fin material extend outward radially from the refrigerant tube in a 360 degree radius. The efficiency of the fins in this type of arrangement varies according to the orientation of the fins with respect to the flow of air. Thus, the fins oriented perpendicular to the flow of air have approximately twice the heat transfer capacity of the spines oriented parallel with the flow of air. The air side heat transfer coefficient is enhanced asymptotically with the inverse of fin perimeter, particularly when the fins members are arranged in a position perpendicular to the direction of air flow. This maximizes the flow of heat between fin and air by minimizing the boundary layer of stagnant air at the fin surface.
In simplistic form the equation for transfer of heat is Q=Ah.DELTA.T, where Q is the heat transfer in BTU/hr, A is the surface area, h is the film heat transfer coefficient, and .DELTA.T is the difference in temperature. The heat transfer coefficient h between the refrigerant and the tube is very high (about 200 to 300 BTU/hr/Degree F.), while the h for air is quite low (from 8 to 30 BTU/hr/Deg F.). From a practical standpoint it is never possible to apply sufficient external surface area to overcome this difference in heat transfer coefficient for the two fluids. However the value of air side heat transfer coefficient can be enhanced significantly by producing the fins in the form of strips with a minimum distance from the leading edge to the leaving leading edge of the strip. Thus, increasing the heat transfer coefficient and reducing fin width minimizes boundary layer depth providing increased heat transfer and also increasing the surface area by the additional edges. The present invention employs a unique fabricating process to improve fin material utilization by arranging all of the fins in an orientation perpendicular to the direction of air flow, and increase the heat transfer coefficient by miniaturizing the fins into strips of minimum depth or width.
Increasing both area ("A") of the heat transfer surface and the heat transfer coefficient ("h") increases the heat transfer capability of the surface significantly. The geometry of the proposed surface positions all of the fins perpendicular to the air stream affording maximum effectiveness for the multi-sheet integral strip fin and refrigerant tube heat exchanger circuit. The integral design maximizes heat flow between the fin and tube and also provides fin rigidity which minimizes handling damage in manufacture and cleaning damage in product use.
For improved performance, an optimization should be achieved between fluid velocity and pressure drop to maximize heat transfer from the refrigerant to the inner wall of the tube or flow conduit. High fluid velocity promotes high heat transfer coefficient but results in high resistance to flow or pressure drop. Since a distinct relationship exists between pressure and temperature (saturation), a high pressure difference as the fluid progresses through the conduit results in a corresponding great temperature difference between the inlet and the outlet which reduces the drive of heat flow to the inner wall of the conduit. In present practice a compromise between the advantage of high velocity and the disadvantage of the resulting pressure drop and temperature change has been accomplished by paralleling or branching the fluid circuits at predetermined points along the circuit. This practice provides marginal optimization of internal heat flow at some predetermined flow rate but leaves much to be desired as rate of flow varies as in heat pump applications. However, an embodiment of the present invention is designed to utilize tapered refrigerant tubes formed integrally within the fins as a novel means to maintain the advantage of high velocity and reduce pressure drop and temperature change within the refrigerant system.